Method for drying particulate materials



May 31, 1966 B. B. AsHBY METHOD FOR DRYING PARTICULATE MATERIALS 2Sheets--SheeifJ 1 Filed NOV. 1, 1963 ATTORNEY.

May 31, 1966 Filed Nov.

TEMPERATURE, F.

B, BjAsHBY METHOD FOR DRYING PARTICULATE MATERIALS 2 Sheets-Sheet"J 2|80 TEMPERATURE PROFILE OF 60 FT* ROTARY DRYER TUBES WHEN CONDENSINGM|XTURE OF METHANOL AND nBUTANOL AT 2O PSIA.

O IO 2O 30 40 50 60 DISTANCE FROM VAPOR INLET FEET.

FIG. 2.

INVENTOR.

BILLY B. ASHBY,

BY 974ml FT/MAM ATTOR NEY.

United States Patent Office 3,253,343 Patented May 31, 1966 3,253,343METHOD FOR DRYING PARTICULATE MATERIALS Billy B. Ashby, Baytown, Tex.,assigner, by mesne assignments, to Esso Research and EngineeringCompany,

Elizabeth, NJ., a corporation of Delaware Filed Nov. 1, 1963, Ser. No.320,898

2 Claims. (Cl. 34-4) This invention relates to the drying of particulatematerials. More particularly, this invention is an improved method fordrying particulate materials by the use of vapors in indirect -heatexchange contact with the particulate materials being dried.

The .usual practice in drying particulate materials is to ow thematerials to be dried through a dryer. An inert gas is normallyintroduced in direct contact with the particulate materials to sweepaway vapor formed during the drying operation. Steam is flowed throughthe dryer in indirect heat exchange contact with the particulatematerials to -be dried. The steam condenses inside tubes running thelength of the dryer. The material to be dried comes in direct contactwith the outside of these tubes. Thus, the heat required for drying isgiven up by the condensing steam, flows by conduction through the tubewalls, and is transferred by conduction and convection from the outsidesurface of the tubes to the wet material. However, with the use ofsteam, the condensing temperature inside the tubes and, thus, the tubesurface temperature is virtually constant along the entire length of thedryer. The drying capacity of a dryer is a function of the temperatureof this heat exchange surface. -In general, the higher the temperature,the higher the drying capacity so long as fouling of the indirect heatexchange contact surfaces does not occur.

In many cases, the maximum temperature permissible is limited by themelting point of the material being dried. The melting point oftendepends on the wetness or percentage of volatiles in the material. Thegreater the liquid content of the material, -the'lower the temperaturemust be to avoid fouling. Since the material entering the upper end ofthe dryer in most drying systems is relatively wet and the materialleaving the lower end is relatively dry, the maximum allowabletemperature Varies considerably along the length of the drier. Forexample, in drying a hydrocarbon solvent from polypropylene, themaxi-mum allowable temperature varies fromv 180 F. at the inlet to 260F. at the outlet.

From the foregoing, it is clear that for certain materials, thetemperature must vary along the length of the dryer (being higher at thelower end) in order that maximum drying capacity may be achieved.

Briefly described, my new method for drying a particulate materialutilizes an inclined tube dryer. The particulate material is fed throughthe dryer from the higher end. A mixture of vapors is fed into the dryertubes lfrom the lower end of the drier in indirect heat exchange withthe particulate material. Each of the components of the vaporous mixture-has a different boiling point. The ratios of the components of thevaporous mixture are controlled `as well as the condensing pressure ofthe vapors in order -to control the temperature gradient along the tubesof the inclined dryer.

The invention, as well as its many advantages, will be furtherunderstood by reference to the following detained description anddrawin-gs in which:

FIG. l is a simplified diagram showing the basic equipment;

FIG. 1A is a sectional view taken along the line 11A-1A of FIG. 1; and

FIG. 2 -is a typical tube-temperature profile when condensing a mixtureof methanol and n-butanol at 20 p.s.i.a.

Referring to FIGS. l and 1A, an inclined tube rotary dryer including arotating drum 1t) is used to dry the wet, particulate material. Aplurality of tubes 12 is mounted within the rotating drum 10. The tubes12 extend longitudinally through a major portion of the rotating drum10.

The wet, particulate material is fed into feed hopper 14 and t-hen intot-he rotating drum by means of -a motor operated screw extruder 18. Thedried, particulate ma terial leaves the dryer at exit 20. The dried,particulate material is conveyed from the outlet 22 by a motoroperatedscrew extruder 24. -If desired, lan inert gas can be used to remove thedried, particulate material.

The mixture of vapors containing components With different boilingpoints is fed into the lower end of the inclined dryer from a reboiler26 by means of a vapor line 28. The lightest component of the vaporousmixture tends to concentrate and condense in the upper end of the tubes12 at a relatively low temperature, while the heaviest component tendsto concentrate and condense in the lower end of the tubes 12 at .ahigher temperature.

The condensate drains down Ithe tubes 12 countercurrent to 4the vaporflow. The condensate is fed back to the reboiler 26 by means of thecondensate line 30. Light material -can be bled from the upper end ofthe tubes through bleed valve 34. It is then condensed in a smallauxiliary condenser and stored for recycling .to the reboiler 26.

The temperature gradient along the length of the dryer is determined,among other things, lby the volatilities of the components in themixture, the length of the `tubes 12, and the amount of light and heavymaterials in the tubes 12. The temperature gradient can be controlledautomatically by:

(l) Bleeding olf light material from the upper end of the dryer;

(2) Adding light material to the reboiler; or

(3) Changing the condensing pressure.

Light materials can be bled off from the upper part of the inclineddryer by means of bleed line 32 controlled by bleed valve 34.

The addition of light material to the reboiler 26 is controlled by anautomatically operated valve 36 in light material line 38. The valve 36is operated by temperature controller 4t). The temperature controller 40is, in turn, connected by means of electrical lead 42, slip ring 44mounted in the rotating drum 10, and electrical lead 46 extending to atemperature-sensing means such as a thermocouple 43 extending into oneof the tubes 12. When the temperature detected by the thermocouple 48exceeds a predetermined set point, the valve 36 is opened slightly toadmit light material to the reboiler 26. When the temperature dropsbelow the set point, the valve 36 is closed.

The pressure of, say, 20 p.s.i.a. within the inclined tubes may be heldrelatively constant by controlling the steam rate to .the reboiler 26.The steam is -fed to the coils S0 of reboiler 26 by means of steam line52 controlled by valve 54. A pressure control 56 is mounted on thereboiler 26. The temperature within the tubes 12 of the inclined dryeris a function of the pressure of reboiler 26 which, in turn, is afunction of the iiow rate of the steam -fed to 4the reboiler.

FIG. 2 is a typical ytube-temperature profile obtained by flowing amixture of methanol and n-but-anol throu-gh a 60-foot rotary drier at 20p.s.i.a. Methanol boils at a temperature of 164 F. at 2O p.s.i.a. Theboiling point of n-butanol .at 20 p.s.i.a. is 259 F. The mixture ofmethanol and nbutanol condensed in the rot-ary drier consists of 45percent methanol and 55 percent n-butanol.

Mixtures of two or more components consisting of vaporous materialsother than methanol and n-butanol can be lused in practicing my newmethod. `For example, a mixture of hexane and Z-methylheptane can beused instead of the mixture of methanol and n-butanol. Hexane boils at170 F. at 18 p.s.i.a., and 2-rnethylheptane boils at 260 F. at 18p.s.i.a.

The particular mixture used in a particular drying operation and thecondensing pressure depend, among other things, on the material to bedried and the temperature prole desired.'

I claim:

1. A method of drying a particulate material in an inclined dryercomprising lthe steps of: feeding the particulate material through thedryer from the higher end; and feeding a mixture of vapors containing atleast two components having different boiling points into the lower endof the inclined dryer and -in indirect heat exchange with -theparticulate material at controlled ratios and controlled condensingpressure to control the temperature ygradient along .the inclined dryer.

2. A method of drying -a particulate material in an inclined dryercomprising the steps of: feeding the particulate material through thedryer from the higher end; feeding a mixture of vapors containing atleast two components having dierent boiling points into the lower end ofthe inclined dryer and in indirect heat exchange with the particulatematerial at controlled ratios and controlled condensing pressure tocontrol the temperature gradient along the inclined dryer; removing thecondensate from the inclined dryer; vaporizing said condensate; andrecycling the vapors to the inclined dryer.

References Cited by the Examiner UNITED STATES PATENTS 2,350,934 6/1944Schutte 34-4x 2,471,325 5/1949 Hickman 34-5 3,174,229 3/1965 stores eta1 34-5 x WILLIAM F. ODEA, Primary Examiner.

JOHN I. CAMBY, Assistant Examiner.

1. A METHOD OF DRYING A PARTICULATE MATERIAL IN AN INCLINED DRYERCOMPRISING THE STEPS OF: FEEDING THE PARTICULATE MATERIAL THROUGH THEDRYER FROM THE HIGHER END; AND FEEDING A MIXTURE OF VAPORS CONTAINING ATLEAST TWO COMPONENTS HAVING DIFFERENT BOILING POINTS INTO THE LOWER ENDOF THE INCLINED DRYER AND IN INDIRECT HEAT EXCHANGE WITH THE PARTICULATEMATERIAL AT CONTROLLED RATIOS AND CONTROLLED CONDENSING PRESSURE TOCONTROL THE TEMPERATURE GRADIENT ALONG THE INCLINED DRYER.